The present invention pertains to fiber optic sensors and, particularly, to fiber optic current sensors which are used in differential protection schemes.
Today's electrical power transmission systems typically include high speed fault protection systems which, for example, operate to preserve the transient stability of the system and to provide better power quality through reduction in voltage sag durations. One type of fault protection system which is used with existing power transmission lines, e.g., those power lines with nominal voltages of 115 KV and greater, is the current differential system, which uses electrical current value information from the power line to determine whether a fault exists. Current differential systems, also known as line differential systems, do not require voltage measuring devices, as they do not use voltage values in their fault determinations and are, therefore, less sensitive to, for example, power swings and sudden load changes in the system than protection systems based on voltage sensors (although, as described below, voltage measurements can be used in conjunction with current measurements to perform, e.g., fault location). These systems operate by, for example, taking a first set of current measurements at one point on the transmission line, transmitting those measurements to a control station or relay, and comparing the first set of measurements with a second set of current measurements that were taken at a second point on the transmission line to determine if a fault exists on the line.
An example of such a line differential protection system is found in U.S. Pat. No. 6,518,767, the disclosure of which is incorporated here by reference. This protection system uses conventional current transformers (CTs) to sense the current in the power lines and provide inputs to the differential protection circuitry. More recently, however, fiber optic current sensors have been developed as alternatives to CT based current sensing systems. Such fiber optic current sensors operate based on the Faraday effect. Current flowing in a wire induces a magnetic field which, through the Faraday effect, rotates the plane of polarization of the light traveling in the optical fiber wound around the current carrying wire. Faraday's law can be stated as:I=HdL  (1)where I is the electrical current, H is the magnetic field and the integral is taken over a closed path around the current. If the sensing fiber is wound around the current carrying wire with an integral number of turns, and each point in the sensing fiber has a constant sensitivity to the magnetic field, then the rotation of the plane of polarization of the light in the fiber depends on the current being carried in the wire and is insensitive to all externally generated magnetic fields such as those caused by currents carried in nearby wires. The angle, ΔΦ, through which the plane of polarization of light rotates in the presence of a magnetic field is given by:ΔΦ=V∫H.dL  (2)where V is the Verdet constant of the fiber glass.
The sensing optical fiber performs the line integral of the magnetic field along its path, which is proportional to the current in the wire, when that path closes on itself. Thus, ΔΦ=VNI where N is the number of turns of sensing fiber wound around the current carrying wire. The rotation of the state of polarization of the light due to the presence of an electrical current can be measured by injecting light with a well-defined linear polarization state into the sensing region, and then analyzing the polarization state of the light after it exits the sensing region. Alternatively, ΔΦ represents the excess phase shift encountered by a circularly polarized light wave propagating in the sensing fiber.
This technology is related to the in-line optical fiber current sensor as disclosed in U.S. Pat. No. 5,644,397 issued Jul. 1, 1997, to inventor James N. Blake and entitled “Fiber Optic Interferometric Circuit and Magnetic Field Sensor”, which is incorporated herein by reference. Optical fiber sensors are also disclosed in U.S. Pat. No. 5,696,858 issued Dec. 9, 1997, to inventor James N. Blake and entitled, “Fiber Optics Apparatus and Method for Accurate Current Sensing” and U.S. Pat. No. 6,188,811 to James N. Blake and entitled “Fiber Optic Current Sensor”, the disclosures of which are also incorporated herein by reference.
Accordingly, it would be desirable to adapt such optical current sensors for usage in, for example, differential protection schemes to monitor transmission lines.